1. Field of the Invention
The present invention relates to a constant-speed driving control device for motor vehicles which detects, from the input of a control switch for the constant-speed driving control device, command signals for executing the setting of vehicle speed for constant-speed driving, the change of a vehicle speed, and the cancelling of the set vehicle speed.
2. Discussion of Background
FIG. 3 shows the constitution of a conventional constant-speed driving control device. In this drawing, numeral 1 is an automotive engine; numeral 2 denotes an intake pipe; numeral 3 denotes a throttle valve installed in the intake pipe 2; numerals 4a and 4b express links connected to the throttle valve 3; numeral 5 represents an accelerator pedal; numeral 6 is an actuator; numeral 7 is a control switch which outputs a command signal by external selecting operation; numeral 8 denotes a vehicle-speed sensor; and numeral 9 is a control unit.
Next, the operation of the constant-speed driving control device will be explained. The control unit 9 judges an instruction from the amount of output voltage from the control switch 7, and computes a controlled variable of the actuator 6 on the basis of an output signal from the vehicle-speed sensor 8 during constant-speed driving or other. The actuator 6 that has received a result of this computation operates to open and close the throttle valve 3 through the link 4a. Here, when a driver operates the control switch 7 to output an instruction signal to cancel constant-speed driving, the throttle valve 3 being actuated by the actuator 6 is stopped. The throttle valve 3 is operated to open and close through the link 4b as the driver depresses the accelerator pedal 5.
FIG. 4 shows the circuit constitution of the control switch 7 and the control unit 9, in which numeral 10 denotes a SET/COAST switch (hereinafter referred to as the S/C switch) which is a command switch for setting constant-speed driving during non-constant-speed driving and for carrying out automatic deceleration during constant speed driving, and numeral 11 denotes a RESUME/ACCEL switch (hereinafter referred to as the R/A switch) which is a command switch for automatic resetting to a target speed during non-constant-speed driving when the vehicle speed is stored in a memory unit and for automatic acceleration during constant-speed driving. The S/C switch 10 and the R/A switch 11 are momentary-type command switches which are constantly kept open and selectively operated to open. With these switches 10 and 11 are connected resistors R.sub.3 and R.sub.2 in series. These sets of serially-connected switches and resistors are connected parallel to each other, with one end being grounded and the other end being connected to an input terminal 18 of the control unit 9 through a terminal area 17. Between the switches 10 and 11 a diode 12 is connected in series. Meanwhile, in the control unit 9, the input terminal 18 is connected with the positive terminal of the comparators 14 to 16 and further connected to a reference power source V.sub.1 through a resistor R.sub.1. This reference power source V.sub.1 is connected to the negative terminal of a comparator 14 through a resistor R.sub.4, and further to the negative terminals of the comparators 15 and 16 and to the ground in order through resistors R.sub.5 to R.sub.7. Also, the output terminals of the comparators 14 to 16 are connected to the ports P.sub.1 to P.sub.3 of CPU 13.
In the constitution of the constant-speed driving control device shown in FIG. 4, when either of the command switches 10 and 11 is closed, the reference voltage V.sub.1 is divided into the resistor R.sub.1 and the resistor R.sub.2 or R.sub.3. The control unit 9 judges a command from the control switch 7 by the amount of the divided voltage. A diode 12 is added to detect the simultaneously closing of the switches 10 and 11 which have been closed by malfunction. Table 1 is a table of operation of the conventional device described above. When the switches 10 and 11 are in the off position, the voltage close to the reference voltage V.sub.1 is used as the input voltage V.sub.IN of the control unit 9; since this voltage is greater than the comparison voltage V.sub.1ref to V.sub.3ref of the comparators 14 to 16 which are divided by the resistors R.sub.4 to R.sub.7, the output of the comparators 14 to 16 becomes H; and therefore an input signal of each of the input ports P.sub.1 to P.sub.3 of the CPU 13 becomes H. Next, when the R/A switch 11 is turned ON, the input voltage becomes ##EQU1## and therefore the input signals at the ports P.sub.1 to P.sub.3 of the CPU 13 will become L, H and H respectively. Subsequently, when the S/C switch 10 is turned ON, ##EQU2## The value of this voltage becomes a medium value of V.sub.2ref and V.sub.3ref. Accordingly the input signals at P.sub.1 to P.sub.3 will become L, L, and H respectively. In case the switches 10 and 11 are simultaneously turned on by malfunction, V.sub.IN becomes close to 0, and the input signals at P.sub.1 to P.sub.3 will all become L. The CPU 13 outputs various command Signals in accordance with various input signals, thereby controlling the actuator 6.
TABLE 1 ______________________________________ CPU input Control switch mode Input voltage V.sub.IN P.sub.1 P.sub.2 P.sub.3 ______________________________________ All OFF V.sub.1 H H H R/A switch ON ##STR1## L H H S/C switch ON ##STR2## L L H Switches 10 and 11 ON 0 L L L ______________________________________
In the conventional constant-speed driving control device described above, however, it is necessary to add the diode 12 to the control switch 7 in order to detect the simultaneous closing of the switches 10 and 11. Also the control unit 9 requires three input comparators and the CPU 13 requires three input ports, which make this device expensive and its construction complicated.